The brake system of a powered vehicle typically includes a service brake for use while the vehicle is in operation and a parking brake for immobilizing the vehicle when it is not in use. Unlike the service brake, the parking brake is not designed to resist the driving force of the vehicle engine and will wear extremely rapidly if it is applied when vehicle is undergoing powered travel. The most usual condition under which that is likely to occur is at start up. The operator may inadvertently forget to release the parking brake prior to shifting the transmission into drive and depressing the accelerator pedal or the like.
Many vehicles are provided with a transmission and brake interlock to avoid damage to the parking brake at start up. As heretofore constructed, the interlock typically neutralizes the transmission, regardless of the position of the operator's shift lever or the like, while the parking brake is engaged. This forces the operator to release the brake as a precondition to initiating powered travel of the vehicle.
While use of the parking brake during vehicle operation is ordinarily undesirable, special circumstances may make it necessary. Most notably, if the service brake should malfunction it may be necessary to rely on the parking brake to slow or stop the vehicle. Because of the limited capacity of the parking brake to substitute for the service brake it would also be desirable to make use of other motion retarding effects to supplement the action of the parking brake in making an emergency stop. Prior interlocks of the kind described above eliminate one source of motion retarding, the vehicle engine, which could otherwise be used for this purpose.